Frequency converter, motor, motor drive system and maintenance method for motor drive system

ABSTRACT

A frequency converter for outputting a power to drive a motor, having: an inverter unit for inverting a d.c. power to an a.c. power; a control unit for controlling the inverter unit; and a housing for supporting at least the inverter unit and control unit, wherein a rise time change unit is provided in the housing, the rise time change unit changes a rise time of a waveform of a voltage output from the inverter unit.

CROSS-REFERENCE TO THE RELATED APPLICATION

This application is a Continuation application of application Ser. No.11/171,354, filed Jul. 1, 2005, which claims priority from Japaneseapplication JP 2004-196188 filed on Jul. 2, 2004, the content of whichis hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to techniques of driving a motor with afrequency converter.

“Influence of Insulation of General Motor Driven with 400 V ClassInverter”, the Japan Electrical Manufacturers' Association, a corporatejuridical person, 1995, March, has the following description that “Whilea motor is driven with an inverter, a surge voltage generated byswitching of the inverter is superposed upon an output voltage of theinverter. If this surge voltage is high, insulation of the motor isadversely affected, resulting in a broken case. This document describesthe inverter surge voltage generation mechanism and its countermeasurein order to avoid beforehand such a case (an excerpt from p. 1, 11. 2 to6).”

This document further gives the following methods as a countermeasurefor insulation damages by a surge voltage during driving a motor with a400 V class inverter (an excerpt from p. 3, 11. 2 and 3):

“4. 2 Surge Voltage Suppressing Method

There are a voltage rise suppressing method and a peak value suppressingmethod in order to suppress a surge voltage.

Output Reactor

If a wiring length is relatively short, a surge voltage can be loweredby suppressing a voltage rise (dv/dt) by installing AC reactors on theoutput side of an inverter (refer to FIG. 4(1)).

However, if the wiring length becomes long, it becomes difficult tosuppress the peak value of a surge voltage in some cases.

Output Filter

A peak value of a terminal voltage of a motor is suppressed byinstalling filters on the output side of an inverter (refer to FIG.4(2)) (an excerpt from p. 3, 11. 6 to 15).”

FIG. 4 in the indications in parentheses (refer to FIG. 4(1) and FIG.4(2)) in the excerpts is the drawing in the document and does notconcern FIG. 4 in this specification.

SUMMARY OF THE INVENTION

Conventional techniques are, however, unsatisfactory in that althoughdeterioration of insulation between windings to be caused by a surgevoltage can be avoided by using AC reactors whose essential object is toreduce noises, the AC reactors are generally large and expensive and inaddition installation space and cost are increased because of additionalcomponents. With the method using output filers or sine wave formingfilters, these filters are large and expensive.

There is another problem of a lowered effective voltage to be suppliedfrom an inverter to a motor, if AC reactors, output filers or sine waveforming filers are used.

In consideration of these problems, the present invention has as itsobjects to improve insulation deterioration between motor winding turnsto be caused by a surge voltage, with compactness, cost reduction andthe like being considered, and to suppress an effective voltage frombeing lowered through insertion of AC reactors, output filters or sinewave forming filters.

In order to achieve the above-described objects, in providing a risetime change unit for reducing a surge voltage influence by changing arise time of the waveform of a voltage to be applied to motor windings,the present invention obtains an inductance of the rise time change unitfor reducing the influence of a surge voltage between motor wiringturns, in accordance with predetermined first and second characteristicsand by the following procedure.

First, the first characteristics are obtained which indicate therelation between a rise time of a voltage to be applied to the motorwindings and a value of a voltage (hereinafter called a surge withstandvoltage) at which partial discharge will not occur between motorwindings.

In accordance with the first characteristics, a rise time (hereinaftercalled a surge voltage suppressing rise time) of a voltage output froman inverter is obtained at which the partial discharge is suppressedfrom being generated between motor winding turns.

Next, the second characteristics are obtained which indicate therelation between inductances of reactors provided between the inverterand motor and a change in the rise time of the voltage output from theinverter.

The inductance satisfying the surge voltage suppressing rise time isobtained from the second characteristics. Reactors having the obtainedinductance are provided between the inverter and motor windings. Thereactor may be wound windings, a coreless reactor without an iron coreor the like. The installation position may be arbitrary positionsbetween the inverter and motor, in a control board on which the inverteris mounted, in a case of the inverter, or in the case of the motor.

The rise time change unit having reactors with the inductance obtainedby the above procedure can reduce the influence of a surge voltage whichcauses insulation deterioration between motor winding turns, with astructure simpler than a conventional structure. The rise time changeunit can be provided in a compact structure and at a low cost.

According to the present invention, a motor drive system having thereliability higher than a conventional reliability can be provided.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating an embodiment of the presentinvention.

FIG. 2 is a diagram showing the position of a surge voltage rushing intoa winding.

FIG. 3A and 3B are diagrams illustrating the generation principle of apartial voltage.

FIG. 4 is a graph showing the relation between a rise time and a surgewithstand voltage.

FIG. 5 is a graph showing the relation between an inductance and a risetime.

FIG. 6 is a table showing the relation among a reactor type, aninductance and upper and lower limits of insulation deterioration start.

FIG. 7 is a diagram showing a system in which a rise time change unit isbuilt in an inverter.

FIG. 8 is a diagram showing a system in which a rise time change unit isbuilt in a control board.

FIG. 9 is a diagram showing a system in which a rise time change unit isbuilt in a motor.

FIG. 10 is a diagram showing a system in which a rise time change unitis built in a motor terminal box.

FIG. 11 is a diagram showing a system in which a rise time change unitis formed by winding cables between a power supply apparatus and amotor, about the outer periphery of the motor.

FIG. 12 is a diagram showing a system in which a rise time change unitis formed by cables between a power supply apparatus and a motor.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will be described.

Prior to describing the embodiments, description will be made ongeneration of a discharge phenomenon called partial discharge betweenmotor winding turns to be cased by a surge voltage.

A surge voltage is considered to be generated in such a manner that whena power is applied from an inverter to a motor via a wiring system, avoltage reflection wave is generated by an impedance difference betweenthe wiring system and motor, and this reflection wave is returned to theinverter and applied also to the motor. Therefore, a theoretical maximumvalue of a surge voltage is twice a maximum value of an applied voltage.

FIG. 2 is a schematic diagram showing the waveform of a voltage appliedbetween stator winding turns of a motor. As shown in FIG. 2, it isassumed that the waveform of a voltage applied to a motor does not risevertically to a voltage value Vp but it shows a voltage rise changehaving a slope and rises to the voltage value Vp after tr second. Avoltage waveform Vin shown in FIG. 2 is the voltage waveform when theapplied voltage reaches a coil winding start position Ms at time t=0.Therefore, as shown in the voltage waveform Vin in FIG. 2, a potentialat the winding start Ms is zero at time point t=0.

The potential at a winding end position Me is also zero at t=0. Apotential difference between the winding start Ms and winding end Me istherefore also zero.

A coil shown in FIG. 2 is an example of a coil of a stator on the fieldside. FIG. 2 shows the coil accommodated in one of a plurality of slotsof the stator. A propagation time T is a time taken for an appliedvoltage waveform at the winding start Ms to reach the winding end Me.

The applied voltage Vin shown in FIG. 2 is assumed that it reaches thewinding start Ms at t=0, propagates to the right in FIG. 2 and reachesthe winding end Me after T second.

FIGS. 3A and 3B show potential changes at the winding start Ms andwinding end Me as the voltage wave shown in FIG. 2 propagates to theright. FIG. 3A shows a shorter rise time, and FIG. 3B shows a longerrise time.

In FIG. 3A, a potential VaMs at the winding start Ms taking zero at t=0takes a voltage value Vp after tra second which is a voltage waveformrise time.

An applied voltage will not reach the winding end Me until t=T and thepotential VaMe at the position of the winding end Me is zero. Therefore,as the potential VaMs at the winding start Ms rises, a potentialdifference (VaMs−VaMe) between the winding start Ms and winding end Merises from zero toward Vp. After t=T, as the potential VaMe at thewinding end Me rises, the potential difference (VaMs−VaMe) takes a flatvalue and then lowers toward zero, showing a trapezoidal change.

The shorter rise time in FIG. 3A has been described above. The longerrise time in FIG. 3B is similar to the shorter rise time and thedescription thereof is omitted.

A potential difference between arbitrary two points of a coil appliedwith a voltage is represented by a percent relative to the appliedvoltage. This is generally called a partial voltage. It is consideredthat as the amplitude of a partial voltage becomes large, partialdischarge occurs between motor winding turns.

With reference to FIGS. 3A and 3B, description will be made on anamplitude of a partial voltage, a rise time and a propagation time.

FIGS. 3A and 3B show a potential difference, i.e., a partial voltage,between the winding start Ms and winding end Me set to arbitrary twopoints of a coil.

As seen from FIGS. 3A and 3B, there is a tendency that the shorter therise time, the larger the partial voltage, and the longer the rise time,the smaller the partial charge voltage. This can be confirmed from thatalthough the maximum value of the potential difference waveform(VaMs−VaMe) between the winding start and winding end is Vp, the maximumvalue of the potential difference waveform (VbMs−VbMe) between thewinding start and winding end is smaller than Vp.

In other words, if an applied voltage rises steeply, the partial voltagebecomes large, whereas if an applied voltage rises gently, the partialvoltage becomes small. This matches the reported case.

Although the relation between the propagation time and partial voltageis not discussed above, there is a tendency that as a propagation timebecomes long, the partial voltage becomes large, whereas as thepropagation time becomes short, the partial voltage becomes small.

Since a propagation speed of the voltage wave is constant, thepropagation time is proportional to a propagation distance. Therefore,as the length of a coil becomes long, the partial voltage becomes large,or as the distance between two points of a coil across which a partialvoltage is measured becomes long, the partial voltage becomes large.

Therefore, in FIGS. 2 and 3A and 3B, the partial voltage is studied byusing the winding start Ms and winding end Me having a maximum length ofa coil as arbitrary two points of the coil, to thereby obtain studyresults of partial voltages higher than the partial voltage between thewinding start and winding end.

Even if the partial voltage between the winding start Ms and winding endMe becomes maximum, the discharge phenomenon does not occur if bothwinding points are spaced more than a discharge start distance. However,since a coil is wound in the slot at random by using a winding machine,there is a case that the winding start Ms and winding end Me arepositioned within the discharge start distance.

Considering this case, the generation of the partial discharge by apartial voltage has been studied assuming that a large partial voltageis generated because the winding start Ms comes near the winding end Me.In other words, under this condition if there is any solution means forsuppressing the partial discharge, the partial discharge can besuppressed even under other conditions and states.

For the simplicity of the above description, although the maximum valueof a partial voltage is set to 100% of the applied voltage, this valueis actually lower than 100%.

Experiment results indicate that the partial voltage is 80% at a risetime of 0.1 μs and almost saturates at 80%. It is known that thissaturation of the partial voltage results from the influence ofcapacitance between winding wires shown in FIG. 2.

The following measurements have been conducted to obtain the firstcharacteristics, on the basis of the above described phenomenon. Forthese measurements, a test equipment was used which can generate avariable amplitude and rise time of an output voltage. A pseud surgevoltage output from the test equipment was applied to a pseud motorwinging. The pseud motor winding is an experimental motor windingsatisfying the conditions of easy occurrence of the partial discharge inwhich a maximum propagation time and a large partial voltage are givenand the winding start Ms and winding end Me are disposed in closeproximity to each other.

By using this test equipment, partial discharge start voltages betweenwindings were measured and the relation was obtained between a rise timeand a partial discharge start voltage across windings by the partialvoltage.

The measurements were conducted by using the test equipment forgenerating an experimental surge voltage and experimental motorwindings. Therefore, the first characteristics were obtained bycorrecting the measurement results by considering various states such asactual application times of the surge voltage and the like, so as torealize the state as if the surge voltage is applied to an actual motor.

Analysis was made on the measurement results of the partial dischargestart voltage and the partial voltage at each rise time. A 400 V class,22 kw motor was assumed to be a standard motor subjected to themeasurement object.

(1) A value of a voltage at which a partial discharge occurs in a motorwinding was measured by setting constant the rise time of an outputvoltage from the test equipment and raising the output voltage value.

(2) The rise time was changed in the range from 0.01 μs to 10 μs in themeasurements (1).

The results of the measurements (1) and (2) are shown in FIG. 4.

It is possible to know from FIG. 4 the voltage value at which thepartial discharge occurs at each rise time. In other words, it ispossible to know the rise time of an applied voltage at which thepartial discharge will not occur. For example, it can be known that therise time is set to 0.5 μs or longer to obtain a surge withstand voltageof 1500 Vp.

The following measurements were conducted to obtain the secondcharacteristics.

For the measurements, a voltage having predetermined value and rise timewas applied to a motor winding and reactors were connected to the inputterminals of the motor windings.

(1) A voltage having a predetermined rise time was applied across motorwindings, and a rise time changed by the inductance of the insertedreactors was measured.

(2) The inductance of the reactors was changed in the range from 0.0001mH to 30 mH in the measurements (1).

The results of the measurements (1) and (2) are shown in FIG. 5.

It can be known from FIG. 5 the inductance of the reactor necessary forobtaining a desired rise time. This relation is used as the secondcharacteristics.

Embodiments of the present invention will be described using the firstand second characteristics.

First Embodiment

In this embodiment, the inductance of a reactor can be calculated easilyon the basis of the above-described measurement results. FIG. 1 is aflow chart illustrating the embodiment method. The flow and procedure ofthe method will be described with reference to FIG. 1.

(Step 1) A target surge withstand voltage for suppressing the partialvoltage is determined from motor specifications and the like.

(Step 2) A rise time for lowering the surge voltage to the target surgewithstand voltage is determined from the first characteristics.

(Step 3) An inductance of a reactor to be inserted for setting the risetime for lowering the surge voltage determined in (Step 2) is determinedfrom the second characteristics.

(Step 4) Reactors having the inductance determined in (Step 3) areinserted between an inverter and motor windings.

In accordance with the flow and procedure described above, theinductance of a reactor capable of reducing the influence of the surgevoltage was 0.01 mH in some cases at the surge withstand voltage of 1500V. The reactor having this inductance may be a core-less coil without aniron core or the like. It has been confirmed from experiments that evena core-less coil can prevent surge by reducing the influence of thesurge voltage and suppressing the partial discharge between motorwindings.

Second Embodiment

FIG. 6 shows the correlation between the inductances of reactors ofvarious types and the inductances of the embodiment capable ofsuppressing insulation deterioration to be caused by a surge voltage.

The inductance in the range (1) shown in FIG. 6 is an inductance ofwiring. In other words, no countermeasure for reducing the influence ofthe surge voltage is made and this range is shown for the comparisonwith other types of reactors. If the inverter and motor are connected bythe inductance in the range (1), insulation deterioration by the surgevoltage may occur.

The inductances in the ranges (2) and (3) shown in FIG. 6 can change therise time of a surge voltage longer so that the influence of the surgevoltage can be reduced.

In the range (3), although the surge voltage influence can be reduced,an effective voltage applied to the motor may be lowered.

In contrast, the range (2) has a small drop of the effective voltage anddoes not pose any problem. The “wound wiring” and “core-less reactor”are light and small in volume, has a small drop of the effectivevoltage, can be made compact and not expensive, and changes less thealready installed facilities. They are suitable for reducing the surgevoltage influence.

Third Embodiment

The above-description has been directed to the product of a standard 22kw motor. Depending upon a motor capacity, the length from the windingstart to winding end of a coil changes. As the winding length changes,the propagation time changes so that the relation between the rise timeand surge withstand voltage changes.

For example, as the motor capacity becomes small, a surge voltagepropagation time in a motor winding becomes short. Therefore, curves ofthe surge withstand voltages shown in FIG. 4 shift to the left and thestandard curve shifts to the curve of “shorter propagation time”. As themotor capacity becomes large, a surge voltage propagation time in amotor winding becomes long. Therefore, curves of the surge withstandvoltages shown in FIG. 4 shift to the right and the standard curveshifts to the curve of “longer propagation time”.

Description will be made on the relation between three types of motors“shorter propagation time”, “standard” and “longer propagation time”.The “standard” in FIG. 4 shows the characteristics of a standard 22 kwmotor. The “shorter propagation time” motor has a capacity smaller thanthat of the “standard” motor and a half of the propagation time of the“standard” motor. The “longer propagation time” motor has a capacitylarger than that of the “standard” motor and a twice of the propagationtime of the “standard” motor.

With reference to FIG. 4, brief description will be made on the relationbetween the surge withstand voltage and rise time relative to each motorcapacity. For example, in order to obtain a surge withstand voltage of2000 Vp, a rise time of 1 μs or longer is required for the “standard”motor. In order to obtain the same surge withstand voltage of 2000 Vp, arise time of 2 μs or longer is required for the “longer propagationtime” motor, and a rise time of 0.5 μs or longer is required for the“shorter propagation time” motor.

The rise time necessary for reducing the surge voltage influence changeswith the motor capacity so that the inductance of a reactor necessaryfor the rise time also changes.

In the above description, the surge withstand voltage is set to 1500 Vp.Since the surge withstand voltage changes with the motor capacity, thenecessary inductance changes in some cases.

If reactors are inserted, a voltage drop may occur. It is not preferableif the effective voltage applied to the motor lowers more than necessaryby inserting the reactors for lowering the surge voltage.

As described above, the inductance of reactors for reducing the surgevoltage influence cannot be determined definitely, but it changes asdescribed above and in accordance with the use state, conditions andapplications of a frequency converter, a motor and a system includingthe frequency converter and motor. The inductance of reactors forreducing the surge voltage influence is defined in the following threeranges, by considering the motor type, the surge withstand voltage, adrop of the effective voltage, a safety factor and the like.

(1) First Range (range of ≧0.04 mH and ≦0.2 mH)

This range is obtained from the measurement results of “longerpropagation time” shown in FIG. 4. It can be understood from FIG. 4 thata surge voltage of 2000 Vp is satisfied at 0.04 mH and a surge voltageof 2300 Vp is satisfied at 0.2 mH.

This range has an upper limit of 0.2 mH and can reduce the surge voltageinfluence for almost all types of motors. There is a tendency that boththe volume and weight of a reactor having an inductance of about 0.2 mHbecome large. However, as compared to a conventional AC reactor, it canbe said the volume and weight are still small.

(2) Second Range (range of ≧0.004 mH and ≦0.04 mH)

This range is obtained from the measurement results of “longerpropagation time” shown in FIG. 4. It can be understood from FIG. 4 thata surge voltage of 1500 Vp is satisfied at 0.004 mH and a surge voltageof 2000 Vp is satisfied at 0.04 mH.

Although this range cannot reduce the surge voltage influence for almostall types of motors as different from the first range, it ischaracterized in that a drop of the effective voltage is as small as itposes no problem. It can be anticipated that if a reactor having aninductance of about 0.004 mH is used, it may become necessary to adjustthe surge voltage 1500 Vp in order to reduce the surge voltageinfluence. However, this range can reduce the surge voltage influencefor many types of motors.

A reactor having an inductance of about 0.04 mH has a small volume andweight so that it is advantageous in compactness and light weight.

(3) Third Range (range of ≧0.0003 mH and ≦0.004 mH)

This range is obtained from the measurement results of “standard” shownin FIG. 4. It can be understood from FIG. 4 that a surge voltage of 1500Vp is satisfied at 0.0003 mH and a surge voltage of 1800 Vp is satisfiedat 0.004 mH.

As compared to the ranges (1) and (2), the volume and weight are madesmaller and this range is characterized in that it is advantageous incompactness and light weight.

If a reactor in these ranges is used, the surge voltage influence can bereduced for the “shorter propagation time” motor and “standard” motor.However, for the “longer propagation time” motor shown in FIG. 4,lowering the surge voltage is more difficult as compared to the ranges(1) and (2). It is anticipated, therefore, that a reactor in the range(3) is not used for the “longer propagation time” but a reactor in theranges (1) and (2) are required to be used. However, the “longerpropagation time” motor has generally a relatively large volume andweight so that it can be said that even if a reactor in the ranges (1)and (2) is used, there is only a slight problem in terms of compactnessand light weight.

A drop of the effective voltage can be reduced more than the case ofusing a reactor in the range (2).

Although the three ranges are defined, the embodiment is not limitedthereto. For example, the ranges may be defined in the following.

(a) A range of ≧0.0003 mH and ≦0.2 mH, (b) a range of ≧0.0003 mH and≦0.04 mH, and (c) a range of ≧0.0003 mH and ≦0.004 mH. These ranges (a)to (c) are defined by paying attention to the volume and weight of areactor determined by the upper limit values. For example, forcompactness, the outer dimension as described with reference to FIG. 6is used as a target.

As described above, the ranges of a reactor of the embodiment may beproperly set in accordance with the range of motor types, a drop degreeof the effective voltage, the volume and weight and the like.

Fourth Embodiment

In FIG. 5, when a reactor is not used (only wirings), the rise time is0.25 μs and its surge withstand voltage is 1400 Vp. If a target surgewithstand voltage is set to 1500 Vp, there is a possibility ofinsulation deterioration between motor windings so that it is necessaryto raise the surge withstand voltage.

An AC reactor conventionally used as a countermeasure for dielectricbreakdown by a surge voltage provides a rise time of 25 μs and can delaythe rise time sufficiently so that insulation deterioration betweenwindings does not occur. However, the system becomes large andexpensive.

If reactors (0.01 mH) of wound windings of the embodiment are inserted,the rise time can be delayed to about 1 μs and the target surgewithstand voltage of 1500 Vp can be cleared sufficiently. A reactorhaving an inductance of 0.01 mH can be realized by a core-less coilwithout requiring a core such as an iron core. For example, aninductance of about 0.01 mH can be obtained by winding a wire nine tosixteen turns in a ring shape having a diameter of about 10 cm, withoutrequiring an iron core or the like. The number of turns, a diameter andthe like can be designed by the following equation (1).L=k*μ*π*(a*a)*(N*N)/b(H)   (1)where each coefficients are:

K: Nagaoka coefficient, μ: =4*π*10⁻⁷, a: coil diameter (m), b: coillength (m), and N=the number of coil turns.

If a three-phase a.c. is used for driving a motor, three ring-shapecore-less coils are necessary each being used for each phase.

Other embodiments of the present invention will be described withreference to FIGS. 7 to 12.

FIG. 7 shows a system in which a frequency converter 1 called aninverter drives a motor 4. A rise time change unit 7 having reactorswith the above-described inductances are accommodated in the frequencyconverter 1.

The frequency converter 1 generally called an inverter has in itshousing 28 a rectifier unit 20 for rectifying and a.c. power from apower supply unit to a d.c. power, a smoothing unit 22 for smoothing theoutput from the rectifier unit 20, an inverter unit 24 for inverting ad.c. power to an a.c. power and a control unit 26 for controlling atleast the inverter unit 24. As compared to the inverter unit 24, controlunit 26 and the like, the conventional AC filter, output filer, and sinewave forming filter have larger volumes and weights and are difficult tobe accommodated in the housing. If they are to be accommodated in thehousing of the frequency converter 1, the shape of the housing 28 isrequired to be changed.

However, the rise time change unit 7 of this embodiment is smaller thanthe conventional AC filters, output filers, sine wave forming filtersand the like. Even the core-less coils having the above-described sizecan lower the surge voltage. The rise time change unit 7 using the coilsof this size can be accommodated in the housing 28 of the frequencyconverter 1 as shown in FIG. 7 without a considerable design change ofthe housing 28. The system for driving a motor by a frequency convertercan be made compact, light, and inexpensive.

FIG. 8 shows an embodiment of a system for driving a motor 4 by using afrequency converter 1 mounted in a control board 2.

In the embodiment shown in FIG. 8, a rise time change unit 7 havingreactors with the above-described inductance is accommodated in thecontrol board 2. As the rise time change unit 7 is installed in thecontrol board 2, installation construction works can be omitted andworks of forming an installation area for the rise time change unit 7and other works are unnecessary.

If the reactors, core-less coils or the like described above are usedfor the rise time change unit 7 of the embodiment, the reactors,core-less coils or the like can be installed in the control board 2relatively easily without greatly changing the shape of a housing frame30 of the control board 2, as shown in FIG. 8.

FIGS. 9 to 11 show embodiments in which a rise time change unit 7 havingreactors with the above-described inductance is integrally formed with amotor 4.

FIG. 9 is a schematic diagram showing a rise time change unit 7 havingthe reactors with the above-described inductance installed in thehousing of a motor 4. FIG. 10 is a schematic diagram showing a rise timechange unit 7 having the reactors with the above-described inductanceinstalled in a terminal box 8 having terminals and the like forsupplying a motor with a drive power output from the frequency converteror the like.

FIG. 11 is a schematic diagram showing an embodiment in which thecore-less coils, reactors or the like as the rise time change unit arewound about an outer periphery of a motor 4. The core-less coils,reactors may be wound about a housing for supporting the motor 4, orthey may be wound about the motor 4 and then the motor is covered with ahousing, i.e., they may be wound inside the housing for supporting themotor 4.

In the embodiments shown in FIGS. 9 to 11, the core-less coils, reactorsor the like as the rise time change unit are installed in the motor orits attachments (such as a terminal box), to thereby reduce the surgevoltage influence. Obviously, the installation position is not limitedonly to the inside of the motor or its attachments, but the rise timechange unit may be installed near or adjacent to the motor or itsattachments. By using the motors of these embodiment, a system forreducing the surge voltage influence can be configured without greatlychanging the system configuration including the frequency converter,control board and the like.

FIG. 12 is a schematic diagram showing the structure that intermediateportions of wiring cables 3 interconnecting a frequency converter 1 anda motor 4 are wound in a coil shape to form a rise time change unit 7having reactors with the above-described inductance. Obviously, inaddition to winding portions of the cables, a discrete rise time changeunit having the core-less coils, reactors or the like may be inserted atdesired positions of the cables 3. In this embodiment, duringmaintenance works such as reducing the surge voltage influence, thediscrete rise time change unit having the core-less coils, reactors orthe like may be involved at intermediate positions of the cables 3.Alternatively, the rise time change unit may be installed atintermediate positions of wiring cables interconnecting a frequencyconverter 1 and a motor 4, when the system having the frequencyconverter 1 and motor 4 is configured.

As described in these embodiment, the rise time change unit 7 can beinstalled in a space sufficient for disposing windings, without greatlychanging the system structure including the control board, frequencyconverter and motor. A surge voltage countermeasure system can beconfigured more easily than a conventional system.

According to the embodiments of the present invention, a surge voltagecan be processed with a simpler structure than that of a conventionalsystem.

The simpler structure may be a structure that reactors with only woundwirings or the like are installed at desired positions between an outputunit of a frequency converter and windings of a motor, or otherstructures.

Table 1 shows comparisons between a standard inverter, a standard ACreactor, a sine wave forming filter, and only wound wirings(embodiment), when the volume and capacity of a standard motor of sometype are set to 100%. TABLE 1 Product Weight Volume Motor 100% 100%Inverter 13% 38% Conventional AC reactors 33% 30% Sine wave 47% 44%forming filters Embodiment Wound 2% 3% windings

Both the weights and volumes of the conventional AC reactors, outputfilters, and sine wave forming filters for suppressing insulationdeterioration by a surge voltage are about 30% and 45%, respectively,relative to those of the standard motor. The weight is about 250 to 350%relative to the standard inverter and the volume is almost the same.Therefore, a system configuration becomes large by inserting the ACreactors, output filers or sine wave forming filters.

In contrast, both the weight and volume of the embodiment, e.g., onlywound wirings, are light and compact at about 2 to 3% relative to thestandard motor. The rise time change unit having this weight and volumecan be accommodated in a control board disposed between an inverter anda motor or in the inside of the inverter or motor. The structure of afacility having the inverter and motor is not necessary to be changedgreatly, realizing compactness and low cost.

An effective voltage can be suppressed from being lowered by insertingAC reactors, output filters, or sine wave forming filters.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. (canceled)
 2. A frequency converter for outputting an a.c. power todrive a motor, comprising: an inverter unit for inverting a d.c. powerto an a.c. power; a control unit for controlling said inverter unit; anda housing for supporting at least said inverter unit and said controlunit, wherein a rise time change unit is provided in said housing, saidrise time change unit changes a rise time of a waveform of a voltage ofthe a.c. power output from said inverter unit, and said rise time changeunit comprises a core-less reactor coil whose inductance value L isdefined by the following equation:L=k*μ*π*(a*a)*(N*N)/b wherein L (henry) is equal to or larger than0.0003 mH and equal to or smaller than 0.2 mH, k is Nagaoka coefficient,μ equals 4*π*10⁻⁷, T is circular constant, a is coil diameter (meter), bis coil length (meter), and N is the number of coil turns.
 3. Thefrequency converter according to claim 2, wherein in a case thatfrequency converter outputs M-phase a.c. power, said rise time changeunit is provided for each phase so that M pieces of rise time changeunits are provided, where M is an integer number not less than two. 4.The frequency converter according to claim 2, wherein in a case thatfrequency converter outputs 3-phase a.c. power, said rise time changeunit is provided for each phase so that 3 pieces of rise time changeunits are provided.